Propylene Recovery in FCC Units via Flash Drum Segmentation
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Solution Overview
Problem
In fluid catalytic cracking (FCC) units, propylene and light ends in the unstabilized gasoline used as lean oil for the primary absorber lead to increased load and losses, making high propylene recovery difficult and uneconomical, especially at higher operating pressures.
Innovation Solution
A process involving a primary absorber feed flash drum to decrease propylene and light ends in unstabilized gasoline, where a portion of the liquid fraction is heated and sent to a flash drum to produce a flashed light ends stream, which is then sent to the wet gas compressor train, and the bottoms liquid stream is further heated and routed to the FCC gas concentration absorber, optimizing propylene and LPG recovery.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of substance
If conventional overhead condensing and receiving steps are used, then propylene can be recovered from FCC fractionator overhead, but the liquid phase carries considerable propylene and light ends to the primary absorber, increasing load and losses
Solution Approach 1:
The invention divides the liquid phase handling into two separate streams: a first portion sent to the primary absorber and a second portion routed to a flash drum for light ends removal. This segmentation allows propylene-rich liquid to be separated from light ends before absorber feed, reducing propylene loss while maintaining simple equipment design
Solution Approach 2:
The invention extracts light ends and excess propylene from the liquid phase by routing a portion of the liquid to a flash drum operated at reduced pressure. The flashed vapor containing light ends and propylene is removed and sent to fuel gas, preventing these components from loading the primary absorber
2Loss of substance
If unit operating pressures at main column receiver are increased, then propylene recovery becomes more difficult, but higher pressures favor more propylene in the liquid phase
Solution Approach 1:
The invention changes the pressure parameter by operating the flash drum at reduced pressure (e.g., 0.5-2.0 atm) compared to the main column receiver. This pressure reduction causes light ends and propylene to flash into vapor phase, separating them from the liquid phase sent to the primary absorber, thereby reducing propylene loss regardless of main column operating pressure
3Loss of substance
If propylene in unstabilized gasoline is reduced, then propylene recovery improves, but additional equipment modifications are required
Solution Approach 1:
The invention makes the flash drum a multi-functional unit that handles both light ends removal and propylene enrichment. The flash drum serves to separate light ends from propylene-rich liquid while also providing a pressure reduction stage, eliminating the need for multiple separate equipment modifications
Solution Approach 2:
The flash drum uses its own vapor-liquid equilibrium characteristics at reduced pressure to automatically separate light ends and propylene from the liquid phase. The process is self-regulating based on pressure and temperature conditions, requiring minimal additional control equipment
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This process enhances propylene and LPG recovery by reducing losses, making the process more economical and feasible for existing FCC units without requiring significant equipment modifications, achieving near 99.6% propylene recovery and reducing the load on the wet gas compressor.
Implementation Method 1
feeding the heated stream to a flash drum to produce a flashed light ends stream and a bottoms liquid stream
Implementation Method 2
feeding a second portion of the liquid fraction obtained in step (iii) to at least one heat exchanger to produce a heated stream
Implementation Method 3
routing the liquid stream to a FCC gas concentration absorber
Data Source
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AI summary
The present subject matter relates generally to processes for propylene recovery. More specifically, the present subject matter relates to processes for enhanced recovery of propylene and liquid petroleum gas (LPG) from the fuel gas produced in fluid catalytic cracking (FCC) units by minimizing the light ends and propylene in the unstabilized gasoline which is used as lean oil for the primary absorber of the FCC gas concentration unit.